Copolymers of maleic anhydride and alkylene oxides wherein the maleic anhydride ring opens to form an ester linkage between the maleic anhydride and the alkylene oxide, thus forming a water-insoluble polyester, are known (Fischer, R. F., "Polyesters from Epoxides and Anhydrides", Journal of Polymer Science, Vol. XLIV, pages 155-172 (1960). Addition of certain tertiary amines during polyesterification is known to yield a perfectly linear polyester with no ether linkages that would result from propylene oxide homopolymerization. However, the copolymers of the instant invention wherein the maleic anhydride ring does not open to form the polyester are not known. It is also known that organic bases, such as pyridine, will affect charge transfer anionic polymerization of maleic anhydride (Wurm, H. et al, "The Reaction of Pyridine with Maleic Anhydride", Makromol. Chem. 180, 1581-1588 (1979). The key to the instant invention, however, it that when alkylene oxides are present during the pyridine-maleic anhydride reaction, the alkylene oxides will react through the usual ring opening sequence with the propagating anion forming a water-soluble copolymer of the starting materials. Termination of the alkoxy anion is accomplished through ester formation either with the backbone anhydride units or with previously unreacted maleic anhydride. The instant process, in addition to providing a novel polymer, provides a non-decarboxylating termination sequence for maleic anhydride copolymeriation. Decarboxylation lowers the scale inhibiting capability of a polymer.
The copolymers of the instant invention have been found useful as scale inhibitors. Generally scale deposits are incrustation coatings which may be formed from a wide variety of simple and complex inorganic salts which accumulate on the metallic surfaces of a water-carrying system through a number of different causes. While the method and compositions of the present invention have been found particularly useful in providing inhibition of calcium carbonate and calcium sulfate scales, inhibition of magnesium hydroxide, calcium fluoride, calcium phosphate and other common scales may also be obtained. Various industrial and commercial water-carrying systems are subject to scale formation problems. Scale is of particular concern in heat exchange systems employing water, such as boiler systems, and once-through and open recirculating water cooling systems.
The water employed in these systems ordinarily will contain a number of dissolved salts, the amount and nature of which will, of course, depend upon the source of the water employed. Thus, the water usually conains alkaline earth metal cations, primarily calcium and magnesium, and such anions as bicarbonate, carbonate, sulfate, silicate, phosphate, oxalate, fluoride, and so forth. Combination products of these anions and cations will precipitate from the water in which they are carried to form scale deposits when the concentration of the anion and cation comprising the combination or reaction product exceed the solubility of the reaction product. Thus, when the concentrations of calcium ion and carbonate and/or sulfate ion exceed the solubility of the calcium carbonate and/or calcium sulfate reaction product, a solid phase of calcium carbonate and/or calcium sulfate will form as a precipitate. Precipitation of the reaction product will continue until the solubility product concentrations of the constitutent ions are no longer exceeded.
Numerous factors may be responsible for producing a condition of supersaturation for a particular reaction product. Among such factors are changes in the pH of the water system, evaporation of the water phase, rate of heat transfer, amount of dissolved solids, and changes in the temperature or pressure of the system.
For boiler systems and similar heat exchange systems, the mechanism of scale formation is apparently one of crystallization of scale-forming salts from a solution which is locally supersaturated in the region adjacent the heating surface of the system. The thin viscous film of water in this region tends to become more concentrated than the remainder of the solution outside this region. As a result, the solubility of the scale-forming salt reaction product is first exceeded in this thin film, and crystallization of scale results directly on the heating surface.
In addition to this, a common source of scale in boiler systems is the breakdown of calcium bicarbonate to form calcium carbonate and magnesium hydroxide, water and carbon dioxide under the influence of heat. For open recirculating cooling water systems, in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat by evaporation of water, the chief factor which promotes scale formation is concentration of solids dissolved in the water by repeated evaporation of portions of the water phase. Thus, even a water which is not scale forming on a once-through basis usually will become scale forming when concentrated two, four, or six times. The formation of scale deposits poses a serious problem in a number of regards. The different types of scale which are formed all possess a low degree of heat conductivity. Thus, a scale deposit is essentially an insulating layer imposed across the path of heat travel from whatever source to the water of the system. In the case of a boiler system, the retarded heat transfer causes a loss in boiler efficiency. Further, the heat insulating scale layer inhibits the normal coolant effect of the boiler water resulting in the boiler tube metal approaching the temperature of the fireside. This results in the tube metal reaching a sufficiently high temperature to cause tube burn-out. In addition to this problem, scale formation facilitates corrosive processes, and a substantial scale deposit will interfere materially with fluid flow. Consequently, scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.